1. Field of the Invention
This invention relates to the electro-deposition of metals and, more particularly, to a method and apparatus for periodically reversing the current in the electro-deposition of metals.
2. Description of the Prior Art
Periodic current reversal has been used to advantage in electrodeposition processes for metals. These processes include the electrowinning, electrorefining and electroplating of such metals as, for example, copper, nickel, zinc, silver, tin, lead, cadmium, gold, platinum and indium, as well as other metals.
In periodic current reversal, the polarity of the current supplied to the electrodes is reversed for short periods of time during the electrodeposition cycle. When the current is in the forward mode, metal deposits on the desired electrode. When the current is in the reverse mode, deplating of deposited metal occurs. The length of the period during which the current is in the reverse mode is shorter than the length of the period during which the current is in the forward mode. Usually, the period for the reverse mode is from 1/100 to 1/10 of the period for the forward mode, although lower as well as higher fractions have been used. The frequency of the reversals usually ranges from 2 to 50 reversals per minute.
The use of periodic current reversal in electro-deposition of metals is disclosed in U.S. Pat. Nos. 1,534,709, 1,956,411, 2,119,936, 2,216,167, 2,451,340, 2,451,341, 2,678,909, 2,951,978, 3,755,113, 3,799,850, 3,864,227, 4,024,035, 4,105,517 and 4,140,596.
Many systems for effecting periodic current reversal are known. One such system employs a common control block, which is connected to a group of electrolytic cells, with a controllable thyristor current rectifier for forward current and a mains-guided inverter that returns energy to the power system for reverse current. A drawback of this system is that only either the rectifier or the inverter is operating at any one time, so that the total system capability is not fully utilized. Moreover, the use of the inverter is subject to losses.
In another system, a current rectifier is used which is switched off during the period of reverse current, the cells are short-circuited and the reverse current passes only because of the energy stored in the cells. Because of the short-circuit currents, an effective control of the process is impossible
In yet another system, as disclosed in U.S. Pat. Nos. 4,024,035 (which issued on May 17, 1977) and 4,105,527 (which issued on Aug. 8, 1978) a reversable electric current of different duration is employed in two groups of cells. The reverse current in one group is supplied by the forward current in the second group. The system comprises a first controllable rectifier, a second controllable rectifier, each rectifier being connected to a group of cells through a common diode switch, and a control block. Both groups of cells and rectifiers are connected in series and in opposite directions with respect to each other. The common diode switch is connected between the common points of the cells and rectifiers. The rectifiers are switched off to dampen the transition processes in the system before the polarity of the current is reversed. Disadvantages of this system are the expense of interconnecting two cell groups, and the interdependence of two cell groups. This requires a shut-down of both groups, when a failure occurs for any reason in either the control block, or in either one of the controllable rectifiers.
Although many advantages can be realized from using periodic current reversal, there are disadvantages. The main disadvantages are: the reductions in the utilization of current during an electro-deposition cycle wherein a forward, or depositon, current and a reverse, or dissolution, current are used. The reductions in current utilization occur for the periods of time during which the forward current is shut off, and the reverse current is applied; for the periods of time needed for redeposition of dissolved metal; and for the periods of time required for effecting the reversal of the polarity of the current from forward to reverse, and from reverse to forward. Other disadvantages are the presence of AC ripple in the DC applied to the cells, and current over-shoot upon current reversal.
The current utilization, C.U., for an electro-depostion cycle may be defined as the ratio between the net current required to obtain the metal deposit during a refining cycle, and the gross current required to obtain the same deposit without current reversal. The C.U. may be expressed approximately as follows: ##EQU1## wherein: i.sub.F =forward current
i.sub.R =reverse current PA1 t.sub.T =total electro-deposition cycle time PA1 t.sub.R =reverse time PA1 t.sub.RS =reverse switching time PA1 t.sub.D ="dead" time PA1 n=number of reversals during one electro-deposition cycle PA1 (i) rectifying a controlled alternating electric current, PA1 (ii) passing the rectified current through a filter reactor, PA1 (iii) passing filtered rectified current to the electro-deposition process and, PA1 (iv) periodically reversing the polarity of the current passing through the electro-deposition process for desired periods of time, the current passing through the filter reactor in the same direction regardless of the polarity of the current passing through the electro-deposition process. PA1 (i) rectifying a controlled alternating electric current, PA1 (ii) passing the rectified current through a filter reactor, PA1 (iii) passing filtered rectified current to the electro-deposition process and, PA1 (iv) periodically reversing the polarity of the current passing through the electro-deposition process for desired periods of time, the current passing through the filter reactor in the same direction regardless of the polarity of the current passing through the electro-deposition process. PA1 (1) a transformer having a primary winding and at least one secondary winding, said secondary winding having either a common point, or a centretap; PA1 (2) an AC power supply connected to the transformer primary winding; PA1 (3) a pair of primary rectifier means consisting of one forward primary rectifier means and one reverse primary rectifier means each connected to a terminal of the transformer secondary winding; PA1 (4) a filter reactor; PA1 (5) a connection linking the forward primary rectifier means to a first end of the filter reactor; PA1 (6) a connection linking the reverse primary rectifier means to a second end of the filter reactor; PA1 (7) one reverse steering rectifier means connected with the first end of the filter reactor; PA1 (8) one forward steering rectifier means connected with the second end of the filter reactor; PA1 (9) A load comprising an electro-deposition process connected between said common point or centre tap of the transformer secondary winding and both the forward and the reverse steering rectifier means; and PA1 (10) control means adapted to switch and control both the primary rectifier means and the steering rectifier means to cause current to flow through the electro-deposition process in either a forward mode, or in a reverse mode as desired.
t.sub.FS =forward switching time
The switching time t.sub.s from full forward to full reverse current or vice versa equals t.sub.FS +t.sub.D +t.sub.RS, the total switching time for one reversal being twice this sum.
The efficiency of the electro-deposition process is directly dependent on the current utilization and any increase in utilization will increase the process efficiency. As the forward current and the reverse current and the forward time and the reverse time as well as the total cycle time usually have fixed values, in order to realize process objectives, only a reduction in switching time will lead to increased current utilization.
In conventional methods for periodic current reversal, the time required for each reversal of current polarity is in the order of 30 to 300 ms (milli seconds), even when using electronic switching. These relatively long switching times are caused by the need to wait until the current comes to zero after switching off the forward current before the reverse current can be switched on. For a given installation, this time is generally constant and is not related to the frequency of reversal. Of course, the higher the frequency of reversals is for an electro-deposition cycle, the higher are the cumulative switching losses, since the switching loss equals the switching time, which is roughly constant, multiplied by the number of times reversal is effected during the electro-deposition cycle time.
Another disadvantage of the prior art processes is the lack of control of AC ripple in the rectified electrical current. The ripple on the DC causes, in many cases, rough growth of depositing metal and dissolution of impurities from the anodes. In processes where a varying current may be used, such as for example, in certain applications of the electrorefining of lead, the harmful effects of ripple increase at lower current values. A third disadvantage is the occurrence of overshoot when the polarity of the current is reversed. Current overshoot has the same deleterious effects as current ripple and should therefore, also be kept as low as possible.
A short reversing or switching time could be instituted by conventional means which, however, would involve the use of a considerable excess transformer voltage which is required to force the current to reverse. This would result in low power factors and expensive transformers, and increased current ripple.
Although the use of a combination of an inductor and a capacitor for filtering the rectified current would alleviate problems associated with AC ripple, the use of a capacitor on a reversing rectifier is also troublesome, because it takes time to reverse the voltage on the capacitor. A high loop gain is required in order to get a fast response in switching when the current polarity is reversed. Low ripple with relatively fast response could be obtained with a 12 or 24 pulse system but the large number of parts and increased complexity of such a system would reduce its reliability and increase its costs.
We have now found that the disadvantages of the known processes and apparatus can be alleviated. Thus, we have found that the AC ripple on the rectified current can be effectively cntrolled at a low level, that losses incurred during the reversing of current during processes for the electrodepostion of metals can be reduced significantly, and that the amount of overshoot of the applied current, after current reversal, can be controlled at low values. These results can be attained by using a filter reactor and rectifier means such that the current polarity is reversed in the load and the rectifiers only, while the same current direction is maintained in the filter reactor. We have also found that the use of a filter reactor is essential, not only to control the ripple on the direct current, but also to obtain smooth metal deposits.